Processes for producing oxygenated products, e.g., aldehydes and/or alcohols, by reaction of olefinically unsaturated compounds with carbon monoxide and hydrogen at elevated temperatures and pressures in the presence of certain catalysts are well known in the art. The aldehydes and alcohols so produced are useful per se and also as intermediates in the production of other valuable compounds. The oxygenated compounds generally correspond to addition of a carbonyl or carbinol grouping to the starting material with simultaneous saturation of the olefin bond. Such processes are generally known under varying names, such as the Oxo-process or the hydroformylation process.
Incorporated herein for background purposes are Pruett et al, U.S. Pat. No. 3,917,661, which deals generally with the hydroformylation of unsaturated organic compounds and Slaugh et al, U.S. Pat. No. 3,239,566, which deals specifically with the hydroformylation of olefins.
A substantial disadvantage in prior art hydroformylation processes is the formation, by a side reaction, of by-products based on hydrogenation rather than hydroformylation of the olefin bond. In commercial experience, substantial amounts of the olefinically unsaturated substrates are simply converted to nearly useless hydrogenation products, instead of the desired oxygenated compounds. A means to suppress or reduce substrate hydrogenation in such processes and the economic value of such a discovery is self-evident. Such a means is the subject of the present invention.
In essence, the present invention comprises the addition of an acidic compound to the hydroformylation reaction mixture, in an amount at least sufficient to suppress substrate hydrogenation. A preferred acid for this purpose is phthalic acid and related compounds.
The use of phthalic acid and related compounds as co-catalysts in the rhodium carbonyl catalyzed hydroformylation of allyl alcohol or its homologs is described in another context applicant's co-pending U.S. patent application, Ser. No. 849,435, filed Nov. 7, 1977. By way of illustration, phthalic acid and related compounds are shown therein to be particularly effective as co-catalysts (used in conjunction with the rhodium carbonyl/triarylphosphine combination) for the oxo tetrahydrofuranylation process. The designed purpose of the phthalic acid in that process was promotion of the conversion of the 2-hydroxytetrahydrofuran intermediate to a stable 2-alkoxytetrahydrofuran (Equation 1). ##STR1##
The phthalic acid has a somewhat rate-retarding effect on the hydroformylation reaction (allyl alcohol) itself, but no particular benefits or deleterious effects in that step of this relatively complicated system were perceived.
With a simpler system, however, a dramatic positive consequence of using the phthalic acid catalyst component in the hydroformylation reaction has been discovered. Application of the phthalic acid/triphenylphosphine/rhodium carbonyl catalyst to the hydroformylation of conventional olefins demonstrates an unprecedented effect--the substrate hydrogenation side reaction characteristic of the Oxo process can be virtually completely suppressed with this combination. This effect is clearly demonstrated in a comparison of the hydroformylation of 1-hexene in the absence and presence of the phthalic acid catalyst modifier. The results are outlined in Equation 2 and 3. A similar result was obtained with 1-decene as the substrate. ##STR2##
The hydrogenation suppression effect is also operable in the allyl alcohol-oxo tetrahydrofuranylation case. In a controlled comparison as above, the extent of direct propanol formation with and without the phthalic acid co-catalyst is 4% and 21% respectively. If there are no offsetting negative effects of the phthalic acid, the yield of 1,4-butanediol obtainable on hydrolysis-hydrogenation of the intermediate(s) should be correspondingly increased. This has been found qualitatively to be the case.